I_v is the luminous intensity in millicandelas as a function of current. I couldn't verify the 630 mcd value at 30 mA so it should be considered "nominal".
The measurements suggest the possibility that LEDs might be used as a convenient reference light source for photographic purposes and other uses. The internal design could be modified to provide a square source of known size and therefore known luminance. Perhaps the contact pad moved to one side and the perimeter of the luminous material covered to produce a convenient photographic source. A flat plastic covering would probably alter the apparent size of the source slightly. Accurate measurements of the characteristics could be supplied with the reference LED for the convenience of the user.